Due to the characteristics of thin profile and low power consumption, liquid crystal displays (LCDs) are widely used in electronic products, such as portable personal computers, digital cameras, projectors, and the like. Generally, LCD panels are classified into transmissive, reflective, and transflective types. A transmissive LCD panel uses a back-light module as its light source. A reflective LCD panel uses ambient light as its light source. A transflective LCD panel makes use of both the back-light source and ambient light.
As known in the art, a color LCD panel 1 has a two-dimensional array of pixels 10, as shown in FIG. 1. Each of the pixels comprises a plurality of sub-pixels, usually in three primary colors of red (R), green (G) and blue (B). These RGB color components can be achieved by using respective color filters. FIG. 2 illustrates a plan view of the pixel structure in a conventional transflective LCD panel. As shown in FIG. 2, a pixel 10 is divided into three sub-pixels 12R, 12G and 12B, and each sub-pixel is divided into a transmission area (TA) and a reflection area (RA).
A typical sub-pixel 12 is shown in FIG. 3. As shown, the sub-pixel 12 has an upper layer structure, a lower layer structure and a liquid crystal layer 190 disposed between the upper layer structure and the lower layer structure. The upper layer comprises a polarizer 120, a half-wave plate 130, a quarter-wave plate 140, a color filter 144 and an upper electrode 150. The upper electrode 150 is made from a substantially transparent material such as ITO (Indium-tin oxide). The lower layer structure comprises an electrode layer having a transmissive electrode 170 and a reflective electrode 160. The transmissive electrode 170 is made from a transparent material such as ITO. The reflective electrode 160 also serves as a reflector and is made from one or more highly reflective metals such as Al, Ag, Cr, Mo, Ti, and AlNd. The lower layer structure further comprises a passivation layer (PL) 180, a device layer 200, a quarter-wave plate 142, a half-wave plate 132 and a polarizer 122. In addition, the transmissive electrode 170 is electrically connected to the device layer 200 through a via 184, and the reflective electrode 160 is electrically connected to the device layer 200 through a via 182.
In the transmission area as shown in FIG. 3, light (indicated by the arrow) from a back-light source (not shown) enters the pixel area through the lower layer structure, and goes through the liquid crystal layer 190 and the upper layer structure. In the reflection area, light encountering the reflection area goes through the upper layer structure and the liquid crystal layer before it is reflected by the reflective electrode 160.
In a typical LCD panel, the upper electrode 150 is connected to a common line. The lower electrodes are connected to a data line via a switching element, such as a thin film transistor, which can be switched on by a gate line signal. The equivalent circuit for a typical LCD sub-pixel is shown in FIG. 4. In FIG. 4, the common line voltage is denoted by Vcom, VT is the voltage level on the transmissive electrode 170 and VR is the voltage level on the reflective electrode 160 (see FIG. 3). CT represents the capacitance in the liquid crystal layer between the upper electrode 150 and the transmissive electrode 170, and CR represents the capacitance in the liquid crystal layer between the upper electrode 150 and the reflective electrode 160. The transmissive electrode 170 is connected to the data line Data m through a switching element TFT-1 and the reflective electrode 160 is connected to Data m through a switching element TFT-2. TFT-1 and TFT-2 are switched on by a gate line signal from the gate line Gate n−1. Typically, one or more charge storage capacitors are fabricated in the device layer 200 and the passivation layer 180 in a sub-pixel 12 so as to maintain the pixel voltage VT in the transmission area and the pixel voltage VR in the reflection area. As shown in FIG. 4, a charge storage capacitor C1 is connected in parallel to CT and a charge storage capacitor C2 is connected parallel to CR.
The sub-pixel structure as shown in FIG. 4 is known as a single-gap structure. In a single-gap transflective LCD, one of the major disadvantages is that transmittance of the transmission area (the V-T curve) and reflectance in the reflection area (the V-R curve) do not reach their peak values in the same voltage range, as shown in FIG. 5. As a result, the reflectance experiences an inversion while the transmittance is approaching its higher value. As shown in FIG. 5, the transmittance starts to peak around 4V but the reflectance is already in decline at about 2.7V.
In order to overcome this inversion problem, a dual-gap design is used in a transflective LCD. In a dual-gap transflective LCD, as shown in FIG. 6, the gap GR in the reflection area RA in the sub-pixel 12′ is about half the gap GT in the transmission area TA. Thus, the thickness of the liquid crystal layer 190 in the reflection area RA is one half the thickness of the liquid layer 190 in the transmission area TA. As such, the transmittance and the reflectance of the LCD are more consistent with each other.
While the optical characteristics of a dual-gap transflective LCD are superior to those of a single-gap transflective LCD, the manufacturing process for controlling the gap in the reflection area in relation to the gap in the transmission area is complex. The production yield for dual-gap transflective LCDs is generally lower than that of single-gap transflective LCDs.
In order to make the transmittance and the reflectance of a single-gap LCD more consistent with each other, it is possible to reduce the voltage potential VR by applying an insulating film over part of the upper electrode. As shown in FIG. 7, the upper electrode of the sub-pixel 12″ comprises two sections: a first electrode section 152 in the reflection area RA and a second electrode section 154 in the transmission area TA. An insulating film 220 is disposed between the first electrode section 152 and the liquid crystal layer 190. The first electrode section 152 can be directly disposed on the substrate 210, but the second electrode section 154 is disposed on top of an intermediate layer 222 in order to make the gap in the transmission area TA substantially equal to the gap in the reflection area RA. A drawback for this type of single-gap transflective LCD is that the manufacturing process for controlling the thickness of the insulating film over the first electrode section 152 is also complex. Furthermore, the insulating film 220 and the intermediate layer 222 must be sufficiently transparent and uniform so as not to affect the optical quality of the display panel.
It is thus advantageous and desirable to provide a method and device to improve the optical characteristics of a single-gap transflective LCD without significantly increasing the complexity in the manufacturing process.